W-CDMA (Wideband Code Division Multiple Access, i.e., the third generation mobile communication system cosponsored by Japan and Europe) has been standardized as a third generation cellular mobile communication scheme by 3GPP (3rd Generation Partnership Project), and cellular mobile communication services using W-CDMA have been sequentially provided. Further, evolved universal terrestrial radio access (hereinafter, “E-UTRA”) and an evolved universal terrestrial radio access network have been considered by 3GPP.
OFDM (Orthogonal Frequency Division Multiplexing) has been proposed as an E-UTRA downlink. Additionally, DFT (Discrete Fourier Transform)-spread OFDM, which is a single-carrier communication scheme, has been proposed as an E-UTRA uplink.
FIG. 9 schematically illustrates an E-UTRA channel structure. An E-UTRA downlink includes a downlink pilot channel, a downlink synchronization channel, a broadcast channel, a downlink control channel, and a downlink shared channel.
An E-UTRA uplink includes an uplink pilot channel, a random access channel, an uplink control channel, and an uplink shared channel.
FIG. 10 illustrates an allocation example of the random access channel, the uplink shared channel, and the uplink control channel, which are included in an uplink frame. The uplink pilot channel is allocated using time-multiplexing in regions of the uplink shared channel and the uplink control channel. The horizontal and vertical axes shown in FIG. 10 denote time and frequency, respectively.
Each small square region shown in FIG. 10 is a time-and-frequency region called a resource unit. In this case, each resource unit is defined by 1.25 MHz in the frequency direction and 1 ms (i.e., 1 TTI (Transmit Time Interval)) in the time direction. The densely hatched region shown in FIG. 10 denotes the random access channel. The lightly hatched region denotes the uplink control channel. The other non-hatched region denotes the uplink shared channel.
Hereinafter, a structure of the random access channel for E-UTRA is schematically explained (see Non-Patent Document 1).
A cyclic prefix and a preamble are allocated in the uplink random access channel. The random access channel included in an uplink frame has a guard time length in the time region in consideration of the cyclic prefix length, the preamble length, and the synchronization timing shift, and has 72 subcarriers in the frequency region. A frame in which the random access channel is allocated is controlled by a base station device based on the number of mobile station devices included in a cell. For example, the random access channel is allocated not for every frame, but at a predetermined frame interval. Frequency allocation of random access channels in a frequency band of a radio communication system is also controlled by the base station device. Multiple random access channels can be allocated in the same frequency band.
On the other hand, resource allocation information concerning the downlink shared channel, a modulation scheme, an encoding rate, HARQ (Hybrid Automatic Repeat reQuest) information, MIMO (Multi-Input Multi-Output) information, a mobile station device identifier or a mobile-station-device group identifier (an identifier common to multiple mobile station devices), and the like are allocated in the downlink control channel included in a downlink frame. Information data, upper layer control information (Layer 3 (L3) message), the preamble number of a preamble, which is transmitted from a mobile station device by means of random access and detected by the base station device, and the like are allocated in the downlink shared channel. Regarding random access, the mobile-station-device group identifier included in the downlink control channel includes a random access identifier RA-RNTI (Random Access-Radio Network Temporary Identifier) indicating that a random access response is allocated in the downlink shared channel. The random access identifier RA-RNTI can be one-to-one correlated with each random access channel (see Non-Patent Document 2). In other words, random access channels having different frame positions are correlated to different RA-RNTIs. Further, random access channels having different frequency positions are correlated to different RA-RNTIs.
There are two types of random access channels having different parameters. Regarding one type of random access channels, 64 kinds of preambles are prepared for each cell. Regarding the other type of random access channels, 16 kinds of preambles are prepared for each cell. A preamble is generated based on a Zadoff-Chu sequence, and is correlated to information including a combination of information items, such as a random ID, a random access reason, pathloss/CQI (Channel Quality Information). For example, a total of 64 kinds of information items including a combination of 32 kinds of random IDs (5 bits) and two kinds of pathloss (1 bit) are correlated to a preamble. The pathloss information indicates whether pathloss of a signal that the mobile station device receives from the base station device is greater or smaller than a threshold. In this case, 32 kinds of preambles are selected from the 64 kinds of preambles based on the measured pathloss. Then, one kind of a preamble, which is to be actually transmitted by means of random access, is selected from the selected 32 kinds of preambles. Alternatively, the radio resource allocation size of a required uplink shared channel is selected based on the measured pathloss/CQI and a kind of information data to be transmitted, and then information including a combination of information concerning the radio resource allocation size and a random ID may be correlated to the preamble.
Hereinafter, a random access procedure (contention-based random access that will be explained later) for E-UTRA is schematically explained (see Non-Patent Document 3). FIG. 11 illustrates a sequence of the random access procedure between the base station device and the mobile station device. Four messages are exchanged between the base station device and the mobile station device.
The mobile station device selects one kind of a preamble from 64 or 16 kinds of preambles, and transmits the selected preamble to the base station device using the random access channel (message 1). The base station device detects the preamble transmitted from the mobile station device by performing correlation calculation between the signal received on the random access channel and a stored preamble, and detects a synchronization timing shift of the detected preamble. Then, the base station device transmits, as a random access response, data including uplink-and-downlink resource allocation information and a random access identifier RA-RNTI that is a random access response identifier to the mobile station device using the downlink control channel. Additionally, the base station device transmits data including the synchronization timing adjustment value, the preamble number of the detected preamble, and a mobile station device identifier C-RNTI (Cell-RNTI) that is unique in a cell to the mobile station device using the downlink shared channel (message 2).
The mobile station device extracts downlink resource allocation information from the downlink control channel including the random access identifier RA-RNTI that is the random access response identifier corresponding to the random access channel used by the mobile station device. Then, the mobile station device receives data on the downlink shared channel based on the downlink resource allocation information. Then, the mobile station device compares the preamble number included in the data received on the downlink shared channel to the preamble number of the preamble transmitted by the mobile station device. If those two preamble numbers are identical, the mobile station device determines that the random access has succeeded. If those two preamble numbers are not identical and if the same preamble number is not detected in the downlink control channel and the downlink shared channel, which are included in another frame within a predetermined time, the mobile station device determines that the random access has failed, and performs random access again. In other words, the mobile station device transmits a preamble, and then continues monitoring whether or not the preamble number corresponding to the transmitted preamble is received. Hereinafter, the predetermined time for determining that the random access has failed is called a “time window.”
If it is determined that the random access has succeeded, the mobile station device transmits, to the base station device, data information (L3 message) and a mobile station device identifier IMSI (International Mobile Subscriber Identity) that is unique in the radio communication system, based on the received random access response, with use of the uplink resource allocated by the uplink resource allocation information, according to the synchronization timing adjustment value (message 3). The base station device having received the data information and the IMSI transmits data including at least the IMSI to the mobile station device in the downlink (message 4). The mobile station device receives the data in the downlink, confirms that the IMSI included in the data is the IMSI of the mobile station device, and thereby determines that the random access has completely succeeded. This operation is called contention resolution. Thus, initial communication between the base station device and the mobile station device is established.
Upon transmission of the message 1 in such random access, the mobile station device randomly selects a random ID. For this reason, if multiple mobile station devices select the same random ID and have the same pathloss, the generated preambles are identical. If each of the mobile station devices transmits the same preamble in the same timing on the random access channel corresponding to the same frequency position, the preambles collide with each other, and thereby the base station device cannot properly detect each preamble. In this case, the base station device does not transmit a random access response to the preamble not properly detected. Since a random access response is not transmitted, the mobile station device, which fails to detect, within the time window, a random access response to the preamble transmitted from the mobile station device, retransmits the preamble and continues retransmission on the random access channel until a random access response is detected, resulting in a delay until random access succeeds.
For the above reasons, a method has been proposed in which upon handover from base station device currently in communication to a different base station device, the base station device prepares a preamble dedicated for the handover (called “dedicated preamble”) and assigns the dedicated preamble to the mobile station device performing the handover in order to maximally reduce a delay until establishment of communication and to reduce a time for disconnection of data communication (see Non-Patent Document 4). This is called non-contention based random access. The aforementioned random access using a preamble randomly selected by the mobile station device is called contention based random access.
Hereinafter, non-contention based random access is explained. FIG. 12 illustrates a sequence of a non-contention based random access procedure between the base station device and the mobile station device. When a channel quality of communication with the base station device degrades, the mobile station device searches a handover-destination base station device. When the handover-destination base station device is determined, the mobile station device communicates the determined information to the base station device currently in communication. The base station device receiving the determined information obtains handover preamble information from the handover-destination base station device targeted by the mobile station device. The handover preamble information includes at least the dedicated preamble number and further includes a dedicated-preamble assignment period, a use-permit start timing, and the like. The base station device in communication with the mobile station device transmits, to the mobile station device, a handover command that is control information for ordering a handover, and the handover preamble information received from the destination base station device targeted by the mobile station device (message 0).
Alternatively, the handover preamble information is communicated to the mobile station device by means of signaling, such as MAC (Medium Access Control signaling), RRC (Radio Resource Control signaling), or the like, which is different from the handover command.
The mobile station device having received the handover command selects a dedicated preamble based on the simultaneously received preamble information, and transmits the selected dedicated preamble to the destination base station device (message 1). The destination base station device having detected the dedicated preamble transmits a random access response including the synchronization timing adjustment value to the mobile station device (message 2). Thus, the mobile station device uses a dedicated preamble, thereby preventing a collision with another mobile station device, preventing a delay caused by the collision, and therefore enabling seamless communication.    [Non-Patent Document 1] 3GPP TS 36.211 V1.0.3 (2007-05), Physical Channels and Modulation (Release 8)    [Non-Patent Document 2] 3GPP TSG-RAN WG2 #58bis, 25-29 Jun. 2007, Orlando, USA “Draft0 minutes of the 58bis TSG-RAN WG2 meeting”    [Non-Patent Document 3] 3GPP TS 36.300 V8.0.0 (2007-03), Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)    [Non-Patent Document 4] 3GPP TSG-RAN2 #58, R2-072338, 7-11 May 2007, Kobe, Japan “36.300 CR0002 Update on Mobility, Security, Random Access Procedure, etc. . . . .”